1. Field of the Invention
The present invention is directed in general to magnetic resonance tomography as employed in medicine for examining patients. The present invention is more specifically directed to a method for the automatic determination of system-specific acoustic or mechanical resonances in an individual manner for each magnetic resonance tomography apparatus. Given knowledge of such resonances, they can be avoided during data acquisition from an examination subject in the MR apparatus by limitations of the system parameters.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully utilized for more than 15 years as an imaging method in medicine and in biophysics. In this examination modality, the subject is disposed in a strong, constant magnetic field. As a result, the nuclear spins of the atoms in the subject, which were previously irregularly oriented, are aligned. Radiofrequency waves can then excite these xe2x80x9corderedxe2x80x9d nuclear spins to a precessional movement. This precession generates the actual measured signal in MRT that is picked up with suitable reception coils. The measured subject can be spatially encoded in all three spatial directions by utilizing non-homogeneous magnetic fields generated respectively by gradient coils.
In one method for generating MRT images, a slice, for example in the z-direction of a Cartesian coordinate system, is first selectively excited. The encoding of the location information in the slice ensues combined phase and frequency encoding with two orthogonal gradient fields that, in the example of a slice excited in the z-direction, are generated in the x-direction and the y-direction by the aforementioned gradient coils. The imaging sequence is repeated N times for different values of the phase encoding gradient, for example Gx, and the magnetic resonance signal is digitized and stored N times in each sequence execution in the presence of the readout gradient Gy. A number matrix (matrix in k-space) with Nxc3x97N data points is obtained in this way. An MR image of the observed slice having a resolution of Nxc3x97N pixels can be directly reconstructed from this dataset by means of a two-dimensional Fourier transformation.
The method allows a free selection of the slice to be imaged, so that tomograms of the human body can be acquired in all directions. As a xe2x80x9cnon-invasivexe2x80x9d examination method, MRT is distinguished first and foremost as a tomographic method in medical diagnostics by a versatile contrast capability. Due to the excellent presentation of the soft tissue, MRT has developed into a method that is often superior to X-ray computed tomography (CT). MRT is currently based on the application of spin echo sequences and gradient echo sequences that enable an excellent image quality with measuring times in the range of seconds through minutes.
Constant technical improvements of the components of MRT devices and the introduction of fast imaging sequences have created an increasing number of medical applications for MRT. Real-time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are examples.
The basic structure of one of the components of such an MRT apparatus is shown in FIG. 9. This component includes superconducting basic field magnet 1 (for example, an axial superconducting magnet with active stray field shielding) that generates a homogeneous basic magnetic field in an interior space. The inside of the superconducting basic field magnet 1 is composed of coils situated in liquid helium. The basic field magnet is surrounded by a double shell cryostat (not shown) that is usually composed of stainless steel. The inner shell that contains the liquid helium and also partly serves as a winding body for the magnet coils is suspended at the outer shell via poorly thermally conductive rods, the outer shell being at room temperature. A vacuum exists between the inner and outer shells. The inner and outer shells are referred to as magnet vessel.
The cylindrical gradient coil 2 is concentrically introduced into the inside of a carrying tube in the interior of the basic field magnet 1 by means of carrier elements 7. The carrying tube is outwardly limited by an outer shell 8 and inwardly limited by an inner shell 9.
The gradient coil 2 has partial windings that respectively generates gradient fields that are proportional to the impressed current and are spatially perpendicular to one another. As shown in FIG. 10, the gradient coil 2 has an x-coil 3, a y-coil 4 and a z-coil 5 that are respectively wound around the coil core 6 and thus generate respective gradient fields in the direction of the Cartesian coordinates x, y and z. Each of these gradient coils 3, 4 and 5 is equipped with its own power supply in order to generate independent current pulses with correct amplitude and at the proper time in conformity with the sequence programmed in the pulse sequence controller.
The radio-frequency resonator (RF coil or antenna; not shown in FIGS. 4 and 5) is situated inside the gradient coil 2. It converts the RF pulses supplied from a power transmitter into an electromagnetic alternating field and subsequently converts the alternating field emitted by the precessing nuclear moment into a voltage supplied to the reception branch.
Since the gradient switching times should be as short as possible, gradient rise rates on the order of magnitude of a few 10 mT/m are necessary. In an extremely strong magnetic field like that generated by the basic field magnet 1 (typically between 0.2 and 1.5 Tesla), strong Lorentz forces occur given such switching events. All system components (housing, covers, shell of the basic field magnet, RF body coil, etc.) that are mechanically coupled to the gradient system move (vibrate) due to the influence of these forces.
Since the gradient coil is almost always surrounded by conductive structures (for example, magnet vessel of stainless steel), the pulsed fields create eddy currents in them that, due to interaction with the basic magnetic field, exert forces on these structures and likewise cause them to move. It is standard in many imaging methods to employ periodically repeated gradient pulses, thereby causing a forced mechanical oscillation of the system to occur. If excitation occurs with periods/frequencies that correspond exactly to one of the natural resonant frequencies that every mechanical system has, resonant amplification of the oscillation (resonance step-up) occurs and the motion amplitudes increase noticeably.
These resonant oscillations of the various MRT apparatus components have a negative influence on the MRT system in many respects:
1. Strong air-borne sound (noise) is generated that represents a nuisance to the patient, the operating personnel and other persons close to the MRT apparatus.
2. The vibrations of the gradient coil as well as of the basic field magnet and their transmission to the RF resonator and the patient bed in the interior of the basic field magnet and the gradient coil contribute to an inadequate clinical image quality that can even lead to misdiagnoses (for example, in functional imaging, fMRI).
3. When the oscillations of the outer shell are transmitted via the poorly thermally conducting rods to the inner shell, or when the superconductor itself is excited to oscillate, then an increased helium evaporation occurs in the inside of the shell, so that an correspondingly greater amount of liquid helium must be replenished, leading to higher costs.
4. High costs also arise due to the necessity of installing a vibration-damping system (similar to an optical table) in order to suppress transmission of the oscillations to the floor.
The excitation of these mechanical or acoustic resonances is dependent on the parameters that define the imaging sequences and including the switching of the gradient pulses. Parameters that excite the gradient-induced mechanical oscillations are, for example, the repetition time TR given fast gradient echo sequences or the echo-echo spacing given echo-planar imaging (EPI sequences).
If these dependencies are known, the parameter set that defines the imaging sequences can be limited to such an extent that an excitation of mechanical resonances no longer occurs given periodic switching of the gradients.
The problem faced by those in the field of MR system design is to measure the dependency of resonance ranges with respect to various sequence parameters.
Conventionally, sound level and helium evaporation rate, for example, are directly determined with suitable measuring instruments as a function of the frequency of a sinusoidal gradient pulse train. Measurements of acoustic pressure and evaporations are manually examined for existing resonance locations and, when these are noted, the software of the system computer is informed of their position (referred to as xe2x80x9ccenter frequencyxe2x80x9d) as well as their width.
Special measuring instruments such as, for example, microphones with appropriate electronics as well as sensitive gas flow meters are required for such a procedure and these are not included in the standard equipment of an MR tomography apparatusxe2x80x94if only for reasons of cost. For this reason, such measurements are very complicated and are done only for a few representative units of each model series. This has the disadvantage that manufacturing tolerances or local peculiarities, as a result of which devices in the same model series may differ, lead to shifts of the center frequency or to a variation of the bandwidth, which is in turn expressed in an increased noise emission and an increased helium evaporation rate. Moreover, the image quality is degradedxe2x80x94as mentioned above.
An object of the present invention is to provide a method with which the sequence parameter-dependency of the resonances can be determined in a simpler way, and wherein this determination can be made for every MRT apparatus.
This object is inventively achieved in accordance with the invention in method for determining acoustic resonances in a magnetic resonance tomography system including the steps of implementing a resonance measurement by applying a number of alternating gradient pulses that have a fixed time spacing relative to one another, and applying an excitation pulse and obtaining one or a plurality of MR signals, evaluating the MR signal of the resonance measurement with reference to at least one parameter characterizing the acoustic resonance of the MRT system, repeating the aforementioned steps with variation of the time spacing of the gradient pulses, forming a resonance curve on the basis of the value of the characteristic parameter of the resonance measurement as function of the varied time spacing, and determining the resonance or resonances of the MRT system from the resonance curve.
In order to suppress additional effects that likewise influence the resonant behavior of the characteristic parameter, the resonance measurement can be associated with a non-resonance exciting reference measurement in an embodiment of the invention. Such a reference measurement is meaningful but not compulsory.
For example, the determination of the resonances can in accordance with the invention ensue automatically by means of suitable software.
In the case of a resonance measurement with reference measurement, it is advantageous to apply the two alternating gradient pulses of the reference measurement and the alternating gradient pulses of the resonance measurement to the same gradient coil axis of the MRT system.
Likewise, the shape of the gradient pulses of the reference measurement is advantageously identical to the shape of the gradient pulses of the resonance measurement.
Advantageously, the excitation pulse in the resonance measurement is only emitted in after a number of gradient pulses. In the case of a resonance of the MRT system, a mechanical excitation of natural oscillations is thereby enabled.
In a version of the inventive method employing a reference measurement, the measurement of the MR reference signal as well as the measurement of the MR signal in the resonance measurement ensues immediately after the excitation pulse.
The parameters characterizing an acoustic resonance of the MRT system of this version can be amplitude, frequency or phase of the MR reference signal or frequency or phase of the resonance signal.
In a further version of the inventive method employing a reference measurement, the respective gradient pulse trains in the reference measurement and in the resonance measurement extend beyond the point in time of the excitation. Additionally, the measurement of the MR reference signal as well as the measurement of the MR signal in the resonance measurement ensue during the further gradient pulse.
The parameters characterizing an acoustic resonance of the MRT of this further version embodiment can be amplitude, frequency, phase or echo time or time interval between the excitation pulse and the occurrence of the echo maximum of the MR reference signal, or of the resonance signal.
In a third version of the inventive method employing a reference measurement, the reference measurement and the resonance measurement are initiated by the same excitation pulse, by the reference measurement ensuing after the emission of the excitation pulse and the resonance measurement following immediately thereafter.
The method is implemented for each gradient coil of the gradient system present in the MRT apparatus.
Another method for determining acoustic resonance in a magnetic resonance tomography system is in accordance with the invention includes the steps of implementing a resonance measurement by applying an excitation pulse and a number of alternating gradient pulses that have the same time spacing, and obtaining a number of MR signals, filling the k-space matrix with the obtained MR signals and transforming the matrix into the image domain, repeating the aforementioned steps with variation of the time spacing of the gradient pulse, forming a resonance curve in which the intensity of image artifacts occurring in a defined region of the image is entered as a function of the varied time spacing, and determining the resonance or resonances of the MRT system from the resonance curve.
Here, as well, the determination of the resonance(s) can inventively ensue, for example, automatically by means of suitable software.
It is again advantageous to implement a reference measurement in addition to the actual resonance measurement by applying two gradient pulses alternating in operational sign following the excitation pulse and before the gradient pulses of the resonance measurement, that have the same fixed time spacing relative to one another as do the gradient pulses of the following resonance measurement, and obtaining one or more magnetic resonance reference signals.
It is likewise advantageous to apply the two alternating gradient pulses of the reference measurement and the alternating gradient pulses of the resonance measurement to the same gradient coil of the MRT system.
The shape of the gradient pulses of the reference measurement should likewise be identical to the shape of the gradient pulses of the resonance measurement.
Advantageously, the reference measurement and the resonance measurement are initiated by the same excitation pulse by the reference measurement ensuing after the application of the excitation pulse and the resonance measurement following immediately thereafter. Both measurements (reference and resonance measurement) thus can be integrated in one measuring event.
In order to assure a uniform filling of the k-space matrix, short phase encoding gradient pulses are respectively activated between the alternating gradient pulses of the resonance measurement.
It is again advantageous to implement this further embodiment of the inventive method for each gradient coil in the MRT system.
The above object also is achieved in accordance with the invention in a magnetic resonance tomography apparatus that is suitable for the implementation any of the above-described methods and variation thereof.
The inventive method has the advantage that it can be implemented within the scope of a system adjustment (during the course of building, maintaining and/or modifying the MRT system) and the resonant behavior of the MRT system that varies over longer time spans thus can be determined in the framework of, for example, a service call.